In the course of receiving telephone calls, subscribers who have more than one local extension frequently find it necessary to shunt (i.e. put to one side or hold in abeyance) a call received on an incoming line until a desired party is called to the telephone or is transferred from one to another local extension. The shunting of such a call is more commonly known as "holding" of a call. For this purpose telephone instruments are provided with a hold circuit activated by a "hold" key or a "hold" button. By manipulating this key or button a subscriber is able to transfer an incoming call to a holding circuit instead of to a local telephone instrument. This holding circuit is essentially a shunting circuit which simulates the electrical characteristics of the subscriber's local telephone instrument. This allows the handset of the telephone initiating the hold to be replaced "on-hook." The hold will be released when any of the extension telephones on the initiating telephone are taken "off-hook."
Present electronic hold and hold release circuits which employ silicon controlled rectifiers (SCR) have problems in reliably holding the line on long loops, failing to drop the line when required, or maintaining excessive leakage current due to failure to cut off SCR holding current effectively.
Additionally, many present electronic hold circuits require a separate power source, usually from the 110 VAC power line, thereby creating a possible fire hazard.